I'm not comfortable with equating internets with time. I think it's more a measure of progress, along some dimension... though not necessarily pertinent to human interests.

A suggestion: one internet is equivalent to the period over which the emotional salience of a set of memes degrades to zero. i.e. - an internet is the life cycle of a family of memes. It's similar to a species; there's no strong dividing line, but sex don't happen between facebook and hotbot._________________Hatred is gained as much by good works as by evil. ~ Ellen Degeneres

so what you are suggesting, then, is for one internet to equate to a chapter in a book... and also, when you say "emotional salience of a set of memes degrades to zero", you mean like it literally has no emotional impact or that "oh, didn't that happen a while ago?" kind of "zero"?

also, please define "set of meme", as my idea of "set of meme" would consist of double rainbow trololo, and rickroll...

Memory implies salience of a sort, but not actionable pertinence. So yeah, an insalient meme is that which you recognize as nostalgic.

A set of memes has no definite border, but the interrelatedness limits its extent. It's like pornography: you know it when you see it._________________Hatred is gained as much by good works as by evil. ~ Ellen Degeneres

So what happens to an internet if the memes evolve?_________________"Your weapons are no match for ours! People of Mars, surrender!"
"Um, this isn't Mars. This is Earth."
"Earth? Earth-with-nuclear-weapons Earth?"
"Yes."
[long pause]
"Friend!"